This allows custom distributions of the agent to register
`HttpServerResponseCustomizer` implementations. When a supported HTTP
server instrumentation starts processing a response, the `onStart`
method of all registered implementations will be invoked with the
`Context` of the SERVER span, an instrumentation-specific response
object and `HttpServerResponseMutator` instance that allows appending
headers to that response.
The intent of this is to allow custom distributions to set a header
containing span context information, such as the trace and span IDs. As
such, the initial implementation only allows appending response headers
and nothing else.
The `HttpServerResponseCustomizer` and related classes are currently in
a subpackage of the `io.opentelemetry.javaagent.bootstrap` package in
`javaagent-extension-api`. This makes them get loaded in the bootstrap
classloader, thus directly accessible from instrumentations. I am not
aware if there is an elegant way to put it in the agent classloader
instead, yet have the same instance accessible from both
`AgentInstaller` and instrumentations.
This also includes Tomcat-specific implementation in order to be able to
demonstrate that it works, and add automated testing of this to
HttpServerTest including one implementation.
a9788a22de